This invention relates to free-space optical telecommunication networks, and more particularly to the calibration and positioning of transceivers and transceiver assemblies used in such networks.
Free-space optical telecommunication offers an attractive alternative to hard-wired or radio communication in certain situations. For example, a telecommunications services provider who wants to enter a new geographical area may have little or no hard-wired plant in that area and may wish to avoid the cost and complexity of installing such plant to serve the new area. Similarly, radio communications resources are limited and regulated, and a new telecommunications services provider may not have sufficient rights to use those resources in a new geographical area.
Free-space optical telecommunication is therefore attractive because it avoids the need for hard-wired plant and because, unlike radio telecommunication, it is essentially unregulated. Optical telecommunication also has the advantage of very large information capacity. Thus optical telecommunications links can support a wide range of telecommunications services such as telephone, video, audio, and computer data transmission.
As described in commonly owned, co-pending U.S. application Ser. No. 08/847,196, filed May. 1, 1997, which is hereby incorporated by reference herein, free-space optical networks may employ substantially unguided, point-to-point, electromagnetic communication between area access points and end users. For example, an area may be served by one or more area access points, each having a transceiver for bi-directional, free-space, line-of-sight, electromagnetic communication with one or more nearby relay points. Relay points have at least two, and in some cases-more than two, free-space optical transceiver assemblies for line-of-sight optical communication with area access points, relay points, and/or end points. An end point is similar to a rely point except that an end point has only one transceiver. Relay point and end point transceivers may be located on the roofs of houses or other buildings in the neighborhood served by an above-mentioned area access point. Users of the network may be located at or near any relay point or end point.
Preferably, at least a fraction of the relay points are reachable via more than one path through the network of relay points for providing alternative communication between two points when direct service is undesired or temporarily unavailable.
A possible problem with providing alternative communication paths involves the calibration and alignment of transceiver assemblies during reconfiguration. For example, if communication between two points is temporarily interrupted, both points must normally reestablish communication with an alternative relay or access point. However, reestablishing communication may be problematic when information that indicates the alternative point""s position is even slightly inaccurate, or when the positioning system itself has inaccuracies.
In fact, the problems associated with reestablishing communication between assemblies are compounded by the fact that both light beams must be aligned in order to establish two-way communication. For example, if the number of possible linking directions associated with a single assembly is represented by the variable N, then the time it takes to find one particular linking direction increases as N increases. It follows that the time it takes for two assemblies to independently find linking directions increases with the square of N. Of course, the number of possible linking directions scales with the solid angle in which the counterpart assembly is located. However, the number of possible linking directions (e.g., N2) associated with two assemblies could become so large that a search for a particular linking direction may become impractically slow.
Therefore, it would be desirable to provide a free-space optical network that can be reconfigured quickly. In particular, it would be desirable to provide a network that could be reconfigured in a time that does not scale as the square of N, even when a relay point or an end point has limited or no knowledge regarding the positions of the alternative points.
Another possible problem of such networks involves safety. Some free-space optical networks could, under certain circumstances, be a direct ocular viewing hazard. For example, in a reconfigurable network, the vertical height of the transceiver assembly and the direction of an electromagnetic beam emitted by the transceiver are adjustable for finding alternative relay, access, or end points. Normally, such alternatives are found using a scanning routine. However, a person may be located in the path of the beam during scanning, which may damage that person""s eyes.
Therefore, it would be desirable to provide a free-space line-of-sight optical network that can be reconfigured safely.
Yet another possible problem associated with free-space, line-of-sight, optical communication involves misalignment. For example, if a transceiver assembly at a relay point were mounted to a roof of a home, the position of that assembly may move when the position of the home moves. Such movement is known to occur periodically with temperature changes that accompany the change of seasons and continuously over a period of many years because the foundation of the home may settle over time. When the position or direction of an assembly changes relative to its counterpart assembly, communication therebetween may degrade, or even be lost, especially when the distance between the assemblies is large and/or the diameter of the optical beam carrying the information is small.
Another problem associated with free-space optical communication involves inaccuracies in the positioning system due to intermittent mechanical slippage and loss of positional data. This problem is compounded by the fact that both beams must be aligned.
Therefore, it would be desirable to provide a reconfigurable, free-space, line-of-sight optical network that is immune to misalignment. In view of the foregoing, it is an object of this invention to provide improved free-space, point-to-point, optical telecommunication.
It is a more particular object of this invention to provide safe, reliable, and accurate telecommunication using a telecommunication network that employs substantially unguided, free-space, electromagnetic radiation between spatially distributed points, even when a particular telecommunications pathway of the network is nonfunctional.
It is another particular object of this invention to provide accurate calibration and rapid alignment and realignment of transceiver assemblies mounted at access, relay, and end points.
These and other objects of the invention are accomplished in accordance with the principles of the invention by providing free-space optical telecommunications methods and apparatus that employs substantially unguided, point-to-point, free-space, electromagnetic communication between transceiver assemblies located at end points, relay points, and access points.
For example, in one embodiment of this invention, a self-calibrating, reconfigurable free-space optical transceiver assembly is provided that includes a transceiver, a calibration retro-reflector, and a control unit. The transceiver should be at least rotatable about a vertical axis and include a transmitter and a receiver. The transceiver should face the vertical axis and the retro-reflector should be at a known rotary position with respect to the transceiver. The control unit moves the transmitter so that it emits a rotating electro-magnetic beam. In operation, a portion of the beam is reflected by the retro-reflector when the transceiver faces the retro-reflector and the receiver detects that portion. Other methods and apparatus for calibrating a transceiver, which do not use a retro-reflector, are also provided.
The self-calibrating assembly may be calibrated according to the following method. In a first step, the height of the transceiver assembly is adjusted to about a predetermined vertical height. In a second step, the assembly is rotated about a vertical axis. And, in a third step, the direction of the transceiver is determined when a position detector detects that the assembly is in a predetermined rotary position. An alternative to adjusting the height of the transceiver assembly according to this invention is adjusting an elevation angle of the assembly to a predetermined elevation angle.
According to another aspect of the present invention, a reconfigurable free-space optical network is provided. The network includes at least a first transceiver assembly and a second transceiver assembly, each of which includes a transmitter, a receiver, and a retro-reflector. The first transmitter is for emitting a first beam and faces a first variable direction. Also, the first receiver and retro-reflector substantially face in that first variable direction. The first retro-reflector is located at a known relative position from the first receiver. Preferably, the first transmitter, receiver and retro-reflector are mounted near one another compared to the distance of the second transmitter assembly. In one preferred embodiment, the retro-reflector is mounted above the transmitter, which is mounted above the receiver. Then, the entire assembly is preferably mounted to a motorized platform that can rotate about a vertical axis and can adjust its elevation angle relative to that vertical axis or a horizontal plane. The second transceiver assembly also includes a second transmitter, a second receiver, and a second retro-reflector, all which face substantially in a second variable direction. The retro-reflectors may be used during reconfiguration.
Accordingly, a method of aligning a first optical transceiver assembly and a second optical transceiver assembly is provided. The method includes (1) providing a retro-reflector at a known relative position from the second assembly so that the retro-reflector is in a line-of-sight of the first assembly, (2) transmitting an electromagnetic beam from the first assembly in a search sector for locating the retro-reflector, (3) receiving at least a portion of the beam by the first assembly when the retro-reflector is located, and (4) orienting at least one of the assemblies in response to the receiving so that the assemblies are at least partially aligned.
In another embodiment, a method for rapidly reconfiguring a first assembly and a second assembly is provided. Each assembly has a transmitter and a receiver, as well as a corresponding retro-reflector that is mounted at a known position so that all three components face substantially in the one direction at any given moment, although that direction may change with as required. The method includes transmitting a first optical beam from the first assembly and a second optical beam from the second assembly. Then, both beams are scanned according to an organized search routine. In a third step, at least a portion of the first beam is received by the first assembly when the first beam is reflected by the second retro-reflector. This only occurs when the first receiver and first transmitter face the second retro-reflector, however small adjustments maybe necessary to optimize alignment and power received by each assembly. And, finally, scanning of the first beam is terminated in response to the receiving step. This ensures that the first beam substantially faces the second assembly until that second assembly finds the position of the first assembly.
In yet another embodiment, a method is provided for reconfiguring first and second assemblies in which the retro-reflector of each assembly points in a direction that is different from the direction of its respective transceiver (including a transmitter and a receiver). The method includes: (1) orienting a second retro-reflector to face substantially in the direction of the first transmitter; (2) scanning a first electromagnetic beam transmitted from the first transmitter; (3) receiving at least a portion of the first beam by the first assembly (e.g., first receiver) when the first beam is reflected by the second retro-reflector; (4) terminating the scanning in response to the receiving; (5) rotating the second assembly so that the second transmitter and the second receiver face substantially in the direction of the first assembly and the second retro-reflector faces in a direction which is not in the direction of the first assembly (e.g., such as in an opposite direction); (6) searching for the first beam with the second receiver until the second receiver receives the first electromagnetic beam; and (7) adjusting at least one of the directions of the first transmitter and the second receiver to maximize the beam intensity received at the second receiver.
A method for safely establishing and maintaining communication between a first optical transceiver assembly and a second optical transceiver assembly is also provided. In a first step, an electromagnetic beam, having a power level, is transmitted from the first assembly to the second assembly when the first assembly faces in a first assembly direction and the second assembly faces in a second direction. In a second step, the power level of the beam is monitored at the second assembly during the step of transmitting. In a third step, a change in said power level is detected. In the fourth step, a cause for the change is identified. And, in a fifth step, it is determined whether the identified cause necessitates adjusting an operational parameter. Examples of such parameters are the first and second assembly directions and the power level of the beam received at the second assembly.
Further features of the invention, its nature and various advantages, will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.